The present invention relates generally to ink jet printers and more specifically to those ink jet printers that utilize a piezoelectric crystal to cause ejection of ink droplets onto a writing surface in response to electrical drive pulses applied to the piezoelectric crystal. Exemplary of such ink jet printers is that described in U.S. Pat. No. 3,747,120, entitled Arrangement of Writing Mechanisms for Writing on Paper with a Colored Liquid. In general, the ink jet printers described in the prior art have failed to limit the number of ink droplets that reach the writing surface in response to each single drive pulse applied to the piezoelectric crystal and this failure have resulted in blurring of the printed information. Limiting to one the number of ink droplets that reach the writing surface in response to each drive pulse is highly desirable in order that the blurring of the printed information may be reduced and the resolution of the printed information may thereby be improved.
Due to the resonant ringing response characteristics of the print head and the ink supply therein, each prior art drive pulse applied to the piezoelectric crystal causes multiple ink droplets to be ejected from the print head. Application of such a prior art drive pulse to the piezoelectric crystal has the immediate effect of causing the piezoelectric crystal to flex into the ink supply within the print head, thereby ejecting the desired droplet of ink. But, the flexing motion of the piezoelectric crystal also causes the ink supply within the print head to ring mechanically for a period of time following the ejection of the desired ink droplet. This resonant ringing motion of the ink within the print head results in the ejection of undesired additional ink droplets from the print head.
U.S. Pat. No. 4,106,032, entitled Apparatus for Applying Liquid Droplets to a Surface by Using a High Speed Laminar Air Flow to Accelerate the Same, describes a prior art attempt to restrict to one the number of ink droplets which reach the writing surface in response to a single drive pulse. This solution attempts to coalesce the multiple ink droplets ejected from the print head into a single droplet before they reach the writing surface by accelerating the ink droplets with a high speed laminar air flow. Tests indicate that, in general, the laminar air flow fails to cause the droplets to coalesce together with the result that the ink droplets reach the writing surface separately, thereby causing blurring of the printed information.
In contrast, the present invention is directed to a circuit for generating an electrical drive pulse which is shaped so as to cause only a single ink droplet to be ejected from the print head in response to each single drive pulse. Since only a single ink droplet is ejected from the print head, only a single ink droplet reaches the writing surface and blurring of the printed information is reduced.
The electrical drive pulse is formed as the composite of two separate electrical waveforms. The first is shaped to cause the piezoelectric crystal to flex in a manner causing the desired ink droplet to be ejected from the print head. The second waveform is shaped to cause the piezoelectric crystal to flex in a motion opposite to the ringing motion of the ink supply within the print head. The effect of the composite waveform on the piezoelectric crystal is twofold. First, a single ink droplet is ejected from the print head. And second, the ringing motion of the ink supply within the print head is suppressed with the result that additional ink droplets are not ejected from the print head.